Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the “activated sludge” process, in which micro-organisms through their growth consume organic compounds. The process necessarily includes sedimentation of the micro-organisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically performed in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Typically, this activated sludge treatment has a conversion ratio of organic materials to sludge of about 0.5 kg sludge/kg COD (chemical oxygen demand) (COD being a common measure of the amount of organic pollutants present), thereby resulting in the generation of a considerable amount of sludge that must be disposed of. The expense for the excess sludge treatment has been estimated at 40-60 percent of the total expense of operation of a typical wastewater treatment plant. Moreover, the conventional disposal method of landfilling may cause secondary pollution problems. Therefore, interest has been growing steadily in methods to reduce the volume and mass of the excess sludge.
Within about the last ten years, Membrane Bio-Reactor (MBR) technology has become available which combines membrane technology with the activated sludge process. In this technique, semipermeable membrane(s) having very fine pores are used to separate water from biologically-reduced pollutants in the water stream. The MBR process is not widely used because of cost and fouling problems. In these systems, ultrafiltration, microfiltration or nanofiltration membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the mixed liquor is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier.
MBRs, de-couple the biological process from the need to settle the biomass, since the membrane filters the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high bacteria loading of 10-30 g/L, 2) extended sludge retention time, and 3) short hydraulic retention time. In a conventional clarifier system, such conditions could lead to sludge bulking and poor settleability.
The benefits of MBR include low sludge production, complete solids removal from the effluent, effluent disinfection, removal of COD, solids and nutrients in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. Disadvantages include aeration limitations, membrane fouling, and membrane cost.
Membrane costs are directly related to the membrane area needed for a given volumetric flow through the membrane, or “flux”. Flux is expressed as liters/hour/m2 (LMH) or gallons/day/ft2 (GFD). Typical flux rates vary from approximately 10 LMH to about 50 LMH. These relatively low flux rates, which are limited largely by fouling of the membranes at higher flow rates, have slowed the growth of MBR systems for wastewater treatment.
The MBR membrane is intended to remove solid particles from a so-called “mixed liquor” which is composed of water, dissolved solids such as proteins, polysaccharides, suspended solids such as colloidal and particulate material, aggregates of bacteria or “flocs”, free bacteria, protozoa, and various dissolved metabolites and cell components. In operation, the colloidal and particulate solids and dissolved organics are deposited on the surface of the membrane. The colloidal particles form a layer on the surface of the membrane called a “cake layer.” Cake layer formation is especially problematic in MBRs operated in the “dead end” mode where there is no cross flow; i.e., there is no flow along the surface of the membrane that would help to keep the cake layer from forming. Depending on the porosity of the cake layer, hydraulic resistance increases and flux declines.
In addition to the cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may be irreversible. Compared to a conventional activated sludge process, floc particle size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases resistance and decreases flux.
Collins et al U.S. Pat. No. 6,926,532 describes use of organic flocculating polymers to enhance the performance of MBRs in the treatment of biological waste, specifically to reduce fouling of mechanical membranes. Collins states that flocculating polymers will not inhibit biological activity if not used to excess.
In other related art, Sly et al U.S. Pat. No. 5,443,729 discloses use of magnetite as a bed material in a fluidized bed bioreactor, that is, a system wherein the water to be treated flows through a bed of granular magnetite with an attached biofilm comprising a colony of alive microorganism, pedomicrobium manganicum, to oxidize and remove manganese from water. The Sly patent shows that magnetite does not inhibit biological activity and makes a suitable MBM for biofilm attachment. The Sly patent states that “magnetite particles used in the . . . water purification process have the necessary density and surface characteristics for a suitable support particle.” There is no mention of use of the magnetic properties of magnetite to prevent solids from leaving the bioreactor by using a magnetic separator.
In summary, the Sly patent shows the significant advantage of a fluidized bed bioreactor to treat water because of its low pressure drop and the high surface area of the bed material. It also shows the suitability of magnetite as a bed material for the growth of a biofilm to treat manganese. As noted, however, the Sly patent only discloses use of one microorganism, pedomicrobium manganicum, to remove one pollutant, manganese. The Sly patent also does not exploit the magnetic properties of magnetite; in particular, Sly does not suggest the use of a magnetic separator to keep the magnetite bed material retained in the fluidized bed bioreactor.
In Australian patent 534 238 to Weiss it was shown that microorganisms attach strongly to magnetite without diminishing their capacity to function microbiologically. Mac Rae and Evans, in “Factors Influencing the Adsorption of Bacteria to Magnetite in Water and Wastewater”, Water Res. 17: 271-277 (1983), and “Removal of Bacteria from Water by Adsorption to Magnetite”, Water Res. 18: 1377-1380 (1984) show that magnetite rapidly adsorbed 95-99% of a variety of microbial cells from aqueous suspensions.